Engine performance, be it diesel or gasoline fueled engines, is greatly affected by variations in altitude at which the engine is operated. Such variations of altitude can affect performance to the point that an engine or vehicle tested may not function properly or may have a lower power rating when used in high-altitude applications. Moreover, manufacturers conduct certification testing in the laboratory to comply with the Environmental Protection Agency's on-highway standards for regulated exhaust emissions and smoke, but the emissions can change significantly under different barometric pressures and ambient air temperatures. In recognition of the potential impacts on air pollution, the Environmental Protection Agency and State of California Air Resources Board have adopted “not-to-exceed” regulations that require manufacturers to also certify engine and vehicle emissions for a range of altitudes and ambient air temperatures, via stationary or mobile laboratory testing
There are several ways in which to simulate variable altitudes in the engine or vehicle laboratory. In one common method, higher altitudes are simulated by restricting airflow to the engine intake. However, this approach yields faulty results since the exhaust of the engine is not similarly adjusted for high altitude conditions. Therefore, the engine gas exchange process and turbocharger gas flow (if equipped) are not correct for the altitude being simulated, and this will significantly effect the performance, fuel consumption and exhaust emissions from the engine. A similar problem exists for engines that are produced at higher altitudes and which must be tested for operation at lower altitudes such as sea level. Since most major engine manufacturers employ certification laboratories that are located at relatively low altitudes, that is less than 2,000 feet above sea level, there is a need to test engines and vehicles at higher altitudes to investigate the impact of altitude on performance and emissions. In most cases, the manufacturer mounts the engine in a vehicle which can then be transported to higher altitudes where the engine is again set up and tested at the high altitude. This results in needless expense to the manufacturer and the ultimate customer, as the altitude conditions can be realistically simulated in the stationary engine and vehicle laboratories. Note however that previous attempts to test engines and vehicles at simulated high altitudes by restricting the flow of air to the engine intake have provided inconsistent results due, in large measure, to the fact the intake and exhaust pressures are not equalized.